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650b to 700c, converting the other way

Short version:
There's lots of info on going from 700c to 650b, I'd like to go the other way, what do I need to do.

Detailed version:
I've found road a tandem that I plan to buy, at a stunning price. It is set up for road/touring at the moment, and I plan to use it for road use, ie nothing over 100 miles, with most rides around 20-30 miles. That said, all of my stuff is 700c, and I want to put narrower tires on it as I have on my road bike. I might be able to find an acceptable 650b, but the availability and pricing isn't the same. I'm aware that standard single rider wheels aren't the same as tandem wheels; please don't comment on that as it isn't what this thread is about.
What I'd like to know is what I'll have to do in order to perform the swap.
I'm not worried about the change in trail due to the wheel size change, so I'm not going to worry about fork geometry.
I know as a fact that the 700c wheels, with inflated tires, fit in the frame with plenty of clearance. I'll have to not mount fenders, and that doesn't bother me at all.
The gearing is going to change from 6sp to 9ps when I do this, and I've done that on a zillion bikes so I also see that as irrelevant to this topic.
The brakes however have me stumped. There are no caliper brake holes so I can't just swap long reach for short reach calipers. The current brakes are center pull cantilever brakes. Because the 700c rim is 19mm larger (radius) I'll need to move the mounts unless someone (hopefully) knows of another method. I'm comfortable removing the current mounts and brazing them on 19mm farther up the stays, and I don't think this will have any detrimental structural effects assuming that I don't take forever brazing them on. However, if there is a more strait forward method, like a bolt on stud relocator or something, I'd like to know about it.

You might be able to swap in some V-brakes, which seem to have more height adjustment than traditional cantilevers. Or, since it is a tandem, you could run disc brakes. That might mean a new fork, but I think they make kits to mount disc brakes on standard rear dropouts. That would also give you nice stopping power, since the bike will have 2 people on it and will be heavy.

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I just noticed that it seems linear pull brakes have a longer distance from the mounting boss to the pad than traditional cantilever brakes (see images below). Can anyone confirm or deny this for me?

I did think about disk brakes, and I like how they stop the streamlined recumbents I've made. However, I'm not sure about the forces this will place on the rear end if I add a mount there. I also don't have disk brake hubs. By V-brakes are you referring to the same thing that I am calling linear pull brakes?

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Quote:

Originally Posted by Retro Grouch

Sure sounds to me like a lot of trouble and expense to avoid installing narrower tires on your existing rims. What additional benefits are you projecting?

I would like the price and availability of 700c tires opposed to the more expensive less available 650b tires.
I have some 700c aero wheels (Hed3 as an example as an example) that I would like to use occasionally on the tandem.
I did a quick search through the QBP catalog and the narrowest 650b tire I could find quickly is a 33, I'd like to run 28mm tires. The only folding tire I could find was much wider, and not a slick.
I have periodically spun out in my 53x11 on my road bike. Smaller wheels are going to decrease my top speed. I don't want smaller than an 11 tooth in the back nor do I want to increase the small ring up front to allow a larger big ring in the front.

As a small additional thought, I think 650b is a function of 650b tires having a reputation for being a soft ride, and tire manufacturers playing into this by making more supple 650 tires than they do 26inch or 650c tires. This therefore adding yet another obscure rim size, and decreasing the availability of similar quality tires in the much more common 26 inch size. I'm aware that 650b is more common in other countries, but it isn't where I buy parts. I think 650b, and 27inch are two sizes we could go on perfectly happy without, although I don't really feel like debating that point now.

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Quote:

Originally Posted by Al1943

+1
I think I'd leave it as is, maybe upgrade the freewheel to 7 speed.
Another option would be to replace the rims with 650c, but on a tandem that may be a downgrade.

Al

It isn't a function of needing 9 gears, so much as a function of having a couple 9sp drive trains laying around, and wanting the 11 tooth gear which I won't get (or a 12 for that matter) on a freewheel. I'd like to avoid 650c for the same reasons that I would like to avoid 650b slicks outlined in the post right after yours.

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Have you checked out the over locknut dimension on the tandem? Most modern tandems are 145mm. Santana's are 160mm. If you have a freewheel rear wheel it might not be that wide. I'd sure check that out before I started making a lot of equipment changing or wheel swapping plans.

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Quote:

Originally Posted by Retro Grouch

Have you checked out the over locknut dimension on the tandem? Most modern tandems are 145mm. Santana's are 160mm. If you have a freewheel rear wheel it might not be that wide. I'd sure check that out before I started making a lot of equipment changing or wheel swapping plans.

It's a long story as to why but I have a 145 spaced rear hub in the parts bin, as well as the needed parts to run a trispoke as a 145mm wheel, with the cassette and wheel center in the correct locations.

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Quote:

Originally Posted by jccaclimber

It's a long story as to why but I have a 145 spaced rear hub in the parts bin, as well as the needed parts to run a trispoke as a 145mm wheel, with the cassette and wheel center in the correct locations.

So are you sure the tandem is 145mm? You said that it's 6-speed. That makes me think that it might be a lot narrower.

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Retro-
Actually, that's a good point. I'll have to measure it, although I've either have, or have easy access to road and mtb hubs from 126 to 145 (excluding 140 which might be an issue). Seeing as it is a steel frame, is there any reason that I shouldn't cold set it out by 5mm if needed the way old steel road frames can be cold set?
jhota-
I'm planning to do the brazing myself if needed, and yes, I have brazed frame parts before.
Actually though, those brakes you linked look like they would be fairly simple to make (I have access to a college machine shop) and I could use a project along those lines.